In the case of a point supported element or flat concrete ceiling known from EP 1 070 800 B1, in each lattice beam of the transverse force and punching shear reinforcement the upper and/or lower bent portions between the diagonal struts project beyond the continuous upper chord and/or the continuous lower chord, also in order to form efficiently acting concrete anchoring zones in the ceiling. The serpentine diagonal strut sections are bent regularly and in each case have a diagonal strut oriented at 90° to the chords and then a diagonal strut inclined by 45° to the chords, such that, in the end region of a lattice beam extending towards the support, the diagonal strut closest to the support produces upper and lower concrete anchoring zones which are spaced equidistantly from the vertical support axis.
The lattice beams known from EP 2 050 887 B1 for transverse force and punching shear reinforcement of element or flat concrete ceilings lack a continuous upper chord. On the other hand, anchoring elements are provided which are located one behind the other in the longitudinal direction of the lattice beam with free intermediate spacings and to which the upper bent portions of the serpentine diagonal strut sections are secured. In one embodiment (FIG. 2c) two adjacent diagonal struts are shown inclined in the same direction and substantially parallel to one another at around 45° relative to the lower chords, such that the upper concrete anchoring zone is offset by a considerable amount in the longitudinal direction of the lattice beam relative to the lower concrete anchoring zone of the same diagonal strut by a very large amount, which corresponds approximately to the lattice beam height.
DE 10 2007 047 616 A1 discloses a lattice beam with two lower chords, a continuous upper chord and two serpentine diagonal strut sections, in which in each case a diagonal strut inclined at 90° relative to the chords follows a diagonal strut inclined at 45°. The concrete anchoring zones formed in the region of the securing points of the diagonal strut inclined at 90° lie above one another without any offset in the lattice beam longitudinal direction.
According to German general building approvals, if lattice beams are used as punching shear reinforcements increase factors result of, for example, 1.25 (Approval Z-15.1-38), 1.6 (Approval Z-15.1-289) and 1.7 (Approval Z-15.1-217) relative to slabs or punching shear reinforcement as a function of lattice beam type. These approvals are based on component testing on portions of ceilings. The increase factors identified are lower than with other known traditional reinforcement systems, such as with double-headed bolts.
Tests with lattice beams as punching shear reinforcement are known from Eligehausen et al. (Beton- and Stahlbetonbau 98 [Concrete and Reinforced Concrete Structures 98], (2003), Issue 6). In these tests steep failure cracks starting from the support edge and pointing away from the support arose in the concrete slab, which the perpendicular bars, close to the support, of the lattice beams intersected only in the upper region or passed through above the lattice beam. The concrete pressure zone in the region of the lattice beam lower chords was severely damaged thereby. The efficacy of the punching shear reinforcement was greatly limited thereby.
With lattice beams according to EP 2 050 887 B1, better reinforcement efficacy and higher increase factors can be achieved relative to the punching shear of concrete slabs than with lattice beams according to EP 1 070 800 B1. However, in modern built structures the requirements for reinforcement efficacy and achievable increase factors relative to concrete slab punching shear may be even higher, and cannot be met with these known lattice beams.